1. Field of the Invention
The present invention relates to a resist underlayer film composition and a patterning process using this.
2. Description of the Related Art
In a recent trend to a higher integration and a higher processing speed of LSI whereby requiring miniaturization of a pattern rule, a lithography using photo-exposure which is currently used as a general-purpose technology is approaching to a limit of resolution inherent to wavelength of a light source.
As to the light source for a lithography used in resist patterning, photo-exposure using a light source of a g-beam (436 nm) or an i-beam (365 nm) of a mercury lamp has been used. As the means for further miniaturization, shift of the exposure light to a shorter wavelength is considered to be effective; and thus, a lithography using as the light source thereof, in place of the i-beam (365 nm), a KrF excimer laser (248 nm), which has a shorter wavelength than the i-beam, especially an ArF excimer laser (193 nm), or further an immersion method using an ArF excimer laser with NA of 1.35 have been used, whereby mass-production using a double patterning process with which the pattern pitch thereby obtained is doubled has been started.
On the other hand, it has been known from the past that a bilayer process is effective to form a pattern having a high aspect ratio on a non-planar substrate; and further, to develop the bilayer resist film by a general alkaline developer solution, a silicone polymer compound having a hydrophilic group such as a hydroxyl group and a carboxyl group is necessary.
As to a chemically amplified positive resist composition of a silicone type, a chemically amplified positive resist composition of a silicone type for a KrF excimer laser using a base resin having a part of a phenolic hydroxyl group of polyhydroxybenzyl silsesquioxane thereof, which is a stable alkaline-soluble silicone polymer, protected with a t-Boc group and being combined with an acid generator was proposed. As for an ArF excimer laser, a positive resist composition based on a silsesquioxane having a cyclohexyl carboxylic acid group thereof substituted with an acid-labile group is proposed. Further, as for an F2 laser, a positive resist composition based on a silsesquioxane having a hexafluoroisopropanol group as a soluble group is proposed. The polymer mentioned above includes in its main chain a polysilsesquioxane moiety including a ladder skeleton formed by polycondensation of a trialkoxy silane or a trihalogenated silane.
As to the resist base polymer having a silicon pendant on its side chain, a silicon-containing (meth)acrylate ester polymer is proposed.
As to the resist underlayer film of the bilayer process, it comprises a hydrocarbon compound capable of being etched with an oxygen gas; and in addition, because it becomes a mask when a substrate thereunder is etched, it needs to have a high etching resistance. In etching by an oxygen gas, it needs to be comprised of only a hydrocarbon, not containing a silicon atom. Further, in order to improve line-width controllability of a silicon-containing resist film in the upper layer and to reduce concavity and convexity of a pattern side wall and pattern fall due to a standing wave, it also needs to have a function as an bottom antirefrective coating; and thus, specifically reflectance from the resist underlayer film to the resist upper layer film needs to be controlled at the level of 1% or lower.
Here, calculation results of the reflectance of film thickness till the maximum of 500 nm are shown in FIG. 3 and FIG. 4. In FIG. 3, a substrate reflectance when the k-value of the resist underlayer film is fixed at 0.3 while the n-value is changed from 1.0 to 2.0 in the vertical axis and film thickness from 0 to 500 nm in the horizontal axis is shown, based on the assumption that wavelength of the exposure light is 193 nm, and the n-value reflectance and the k-value reflectance of the resist upper layer film are 1.74 and 0.02, respectively. When the resist underlayer film for the bilayer process having film thickness of 300 nm or more is assumed, an optimum value exists in the n-value range of 1.6 to 1.9 to have the reflectance of 1% or less, the range being the same level or a slightly higher as compared with the resist upper layer film.
In FIG. 4, reflectance when the n-value of the resist underlayer film is fixed at 1.5 while the k-value is changed from 0 to 0.8 is shown. When the resist underlayer film for the bilayer process having film thickness of 300 nm or more is assumed, reflectance of 1% or less is possible in the k-value range of 0.24 to 0.15. On the other hand, the optimum k-value of the bottom antirefrective coating for a monolayer resist used in a thin film of about 40 nm is 0.4 to 0.5, which is different from the optimum k-value of the resist underlayer film for the bilayer process used in the film thickness of 300 nm or more. In the resist underlayer film for the bilayer process, it is shown that a resist underlayer film having further lower k-value, namely a further transparent resist underlayer film is necessary.
However, etching resistance of an acrylate ester is lower as compared with a polyhydroxy styrene in substrate etching, and top of it, to lower the k-value, considerable amount of the acrylate ester must be copolymerized; and as a result, the resistance in substrate etching is substantially deteriorated. The etching resistance has an effect not only on etching rate but also on surface roughness after etching. Worsening of surface roughness after etching which is caused by copolymerization of the acrylate ester became a serious problem.
On the other hand, a trilayer process having a laminate comprising a resist upper layer film of a monolayer resist not containing a silicon, a silicon-containing resist intermediate film thereunder, and a resist underlayer film of an organic film thereunder is proposed. Generally, a monolayer resist has better resolution than a silicon-containing resist; and thus, in the trilayer process, the monolayer resist having high resolution can be used as an exposure imaging layer. As to the resist intermediate film, a spin-on-glass (SOG) film is used, whereby many SOG films are proposed.
Here, optimum optical constants of the underlayer film to suppress the substrate reflection in the trilayer process are different from those in the bilayer process. There is no difference between the bilayer process and the trilayer process in the object to suppress the substrate reflection as much as possible, specifically to the level of 1% or lower; but the antireflective effect is afforded to only the resist underlayer film in the bilayer process, while in the trilayer process the antireflective effect can be afforded to any one of the resist intermediate film and the resist underlayer film or both.
A material for a silicon-containing film afforded with the antireflective effect has been proposed. Generally, the antireflective effect is higher in the bottom antirefrective coating of a multilayer than in that of a monolayer; and thus, this is widely used as the bottom antirefrective coating of an optical material in industry. A high antireflective effect can be obtained by affording the antireflective effect to both the resist intermediate film and the resist underlayer film.
If a function of the bottom antirefrective coating can be afforded to the silicon-containing intermediate film in the trilayer process, it is not necessary to afford the highest effect as the bottom antirefrective coating to the resist underlayer film as in the case of the bilayer process. In the trilayer process, the resist underlayer film is required to have a high etching resistance during the substrate processing rather than to have the effect as the bottom antirefrective coating.
Because of this, a novolak resin containing many aromatic groups thereby having a high etching resistance has been used as the resist underlayer film for the trilayer process.
Here, the substrate reflectance is shown in FIG. 5 when the k-value of the resist intermediate film is changed.
When a low value of 0.2 or less is made as the k-value of the resist intermediate film and the film thickness is set appropriately, sufficient antireflective effect with the level of 1% or lower can be obtained.
Usually, to suppress the reflection to the level of 1% or lower in the bottom antirefrective coating with the film thickness of 100 nm or less, k-value needs to be 0.2 or more (see FIG. 4); but in the resist intermediate film of the trilayer process in which a certain degree of reflection can be suppressed by the resist underlayer film, the k-value of less than 0.2 becomes the optimum value.
Next, change of reflectance when both film thicknesses of the resist intermediate film and of the resist underlayer film are changed is shown in FIG. 6 and FIG. 7 for the cases with the k-value of the resist underlayer film being 0.2 and 0.6.
The resist underlayer film having k-value of 0.2 in FIG. 6 assumes the optimum resist underlayer film for the bilayer process, while k-value of 0.6 of the resist underlayer film in FIG. 7 is nearly equal to the k-value of a novolak or a polyhydroxy styrene at 193 nm.
Film thickness of the resist underlayer film changes with topography of the substrate, but film thickness of the resist intermediate film hardly changes so that application thereof for coating may be effected with the predetermined film thickness.
Here, the resist underlayer film having a higher k-value (the case of 0.6) can suppress the reflection to the level of 1% or lower with a thinner film thickness. In the case that k-value of the resist underlayer film is 0.2 with film thickness thereof being 250 nm, film thickness of the resist intermediate film needs to be thicker to obtain reflectance of 1%. However, when film thickness of the resist intermediate film is increased as mentioned above, burden to the uppermost resist film during dry etching at the time of processing of the resist intermediate film is large; and thus, this is not desirable.
FIGS. 6 and FIG. 7, showing reflection in dry photo-exposure with NA of the exposure instrument's lens being 0.85, show that reflectance of 1% or less can be obtained regardless of k-value of the resist underlayer film by optimizing n-value, k-value, and film thickness of the resist intermediate film for the trilayer process. However, NA of the projection lens is beyond 1.0 due to an immersion lithography so that angle of an incident light not only to the resist but also to the bottom antirefrective coating under the resist becomes shallower. The bottom antirefrective coating suppresses reflection not only by absorption by the film itself but also by compensation action due to a light interference effect. A slant light has a smaller light interference effect thereby leading to larger reflection. Among the films in the trilayer process, a film playing an antireflection role by using the light interference is the resist intermediate film. The resist underlayer film is too thick to use the interference action so that there is no antireflective effect by compensation action due to the light interference effect. Reflection from the resist underlayer film surface needs to be suppressed; and for this, k-value of the resist underlayer film needs to be less than 0.6 and n-value thereof needs to be nearly equal to that of the upper resist intermediate film. Excessively high transparency due to excessively small k-value generates reflection from the substrate; and thus, optimum value of k-value is in the range of about 0.25 to about 0.48 in the case of NA of the immersion exposure being 1.3. As to the re-value, the target value thereof in both the intermediate film and the underlayer film is nearly equal to 1.7 of the resist n-value.
A benzene ring has strong absorption; and thus, k-value of a cresol novolak and a polyhydroxy styrene is more than 0.6. A naphthalene ring is one example among those having higher transparency at 193 nm and higher etching resistance than a benzene ring. For example, resist underlayer films having a naphthalene ring and an anthracene ring are proposed in the Patent literature 1. According to our measurement, k-values of the naphthol-cocondensed novolak resin and of the polyvinyl naphthalene resin are in the range of 0.3 to 0.4. In addition, n-values of the naphthol-cocondensed novolak resin and of the polyvinyl naphthalene resin at 193 nm are low; n-value of the naphthol-cocondensed novolak is 1.4, while that of the polyvinyl naphthalene resin is even as low as 1.2. For example, an acenaphthylene polymer shown in the Patent literature 2 and the Patent literature 3 has n-value of 1.5 and k-value of 0.4 at 193 nm, which are nearly equal to the target value. An underlayer film having a high n-value and a low k-value thereby having low transparency, while having a high etching resistance, is wanted.
A resist underlayer film composition having a bisnaphthol group is proposed in the Patent literature 4. This has both n-value and k-value nearly equal to the target values whereby having characteristic of excellent etching resistance.
In the case that there is a level difference on the founding substrate to be processed, this different level needs to be made flattened by the resist underlayer film. By flattening the resist underlayer film, variance of film thickness of the resist intermediate film formed thereon and of the photoresist film, which is the resist upper layer film, can be suppressed so that a focus margin in lithography can be made larger.
However, in the amorphous carbon underlayer film formed by a CVD method using such a raw material as a methane gas, an ethane gas, and an acetylene gas, to fill up the different level so as to make it flat is difficult. On the other hand, formation of the resist underlayer film by a spin coating method has a merit that concavity and convexity of the substrate can be filled up. Further, in order to improve a fill-up property of an application-type material, a method in which a novolak having a low molecular weight and a wide molecular weight distribution and a method in which a low-molecular weight compound having a low-melting point is blended to a base polymer are proposed.
It is well known that a novolak resin is cured by crosslinking intermolecularly only by heating. Here, a crosslinking mechanism, in which a phenoxy radical is generated in the hydroxyl group of a cresol novolak by heating and this radical is then migrated to the methylene connecting group in the novolak resin by resonance whereby these methylene groups are crosslinked therebetween by radical coupling, is reported. In the P Patent literature 5, a patterning process using the underlayer film whose carbon density is increased by a dehydrogenation reaction or a dehydration condensation reaction of a polycyclic aromatic compound such as a polyarylene, a naphthol novolak, and a hydroxyl anthracene by heating is reported.
A glass-like carbon film is formed by heating at temperature of 800° C. or higher (nonpatent literature 1). However, in view of the effect to device damage and wafer deformation, upper temperature limit of heating in lithography wafer process is 600° C. or lower, or preferably 500° C. or lower.
As the process line width narrows further, a phenomenon that the resist underlayer film twists or bends during the time of etching of the substrate to be processed by using the resist underlayer film as a mask is reported (nonpatent literature 2). A phenomenon that a hydrogen atom of the resist underlayer film is displaced with a fluorine atom during the time of substrate etching by a fluorocarbon gas is shown. It may be supposed that surface of the resist underlayer film is changed to a sort of Teflon (registered trade name) so that the underlayer film swells due to increase of its volume and the glass transition temperature thereof is lowered to cause twisting of a finer pattern. In the foregoing reference literature, it is shown that twisting can be avoided by using a resist underlayer film having low hydrogen content. The amorphous carbon film formed by a CVD method is very effective to avoid twisting because hydrogen atoms in the film can be made extremely small. However, because the CVD method is poor in a fill-up property of the level difference as mentioned before, and in addition, in view of cost of the CVD instrument and the footprint thereof, introduction thereof is sometimes difficult. If the twisting problem can be solved by the underlayer film composition which can form a film by coating, especially by a spin coating method, it has a large merit in simplification of the process and the instrument thereof.
A study of a multilayer process to form a hard mask on the resist underlayer film by a CVD method is underway. In a silicon hard mask (silicon oxide film, silicon nitride film, and silicon oxynitride film) too, an inorganic hard mask formed by a CVD method and so on has higher etching resistance than a hard mask formed by a spin coating method. In addition, there is a case that a substrate to be processed is of a low dielectric film thereby polluting the photoresist (poisoning); in this case, the CVD film has a higher effect as a shielding film to avoid the poisoning.
In view of the above, a study is underway as to the process in which the resist underlayer film is formed by spin coating to make it flat and then an inorganic hard mask intermediate film as the resist intermediate film is formed thereon by a CVD method. When the inorganic hard mask intermediate film is formed by a CVD method, heating of the substrate at the lowest temperature of 300° C., or usually at 400° C., is considered to be necessary especially to form a nitride film. Accordingly, when the resist underlayer film is formed by a spin coating method, heat resistance of 400° C. is necessary; however, not only a usual cresol novolak and naphthol novolak but also a highly heat-resistant fluorene bisphenol cannot withstand the heating at 400° C., whereby resulting in large film loss after heating. In view of the above, a resist underlayer film which can withstand high temperature heating during formation of the inorganic hard mask intermediate film by a CVD method is wanted.
Because of problems of film loss and resin deterioration after heating caused by poor heat resistance as mentioned above, in the past, heat treatment of the resist underlayer film composition has been carried out usually at 300° C. or lower (preferably in the range of 80 to 300° C.). However, problems of film loss after treatment by a solvent and of pattern twist during the time of substrate etching have not been solved yet.
As discussed above, a material to form a resist underlayer film having optimum n-value and k-value as the bottom antirefrective coating, an excellent fill-up property, excellent etching resistance and solvent resistance, and heat resistance to withstand high temperature during formation of the inorganic hard mask intermediate film by a CVD method and so forth, while not twisting during the time of substrate etching is wanted; and wanted also is a patterning process for it.
A positive resist having a truxene structure is proposed (Patent literature 6). A resist based on a truxene having a hydroxyl group thereof substituted with an acid-labile group is introduced as an EB resist and an EUV resist having an excellent etching resistance. Among underlayer-forming materials having a plurality of etching resistant bisphenols, a truxene bisphenol compound is shown (Patent literature 7); and thus, a truxene compound is receiving an attention.    [Patent literature 1] Japanese Patent Laid-Open Publication No. 2002-14474.    [Patent literature 2] Japanese Patent Laid-Open Publication No. 2001-40293    [Patent literature 3] Japanese Patent Laid-Open Publication No. 2002-214777    [Patent literature 4] Japanese Patent Laid-Open Publication No. 2007-199653    [Patent literature 5] Japanese Patent No. 3504247    [Patent literature 6] Japanese Patent Laid-Open Publication No. 2008-76850    [Patent literature 7] Japanese Patent Laid-Open Publication No. 2006-285075    [nonpatent literature 1] Glass Carbon Bull. Chem. Soc. JPN., 41(12), 3023-3024 (1968)    [nonpatent literature 2] Proc. of Symp. Dry Process (2005), p 11.